In chemical processes liquid-liquid extractions are often used to transfer a solute dissolved in a first liquid to a second liquid which is essentially immiscible with the first liquid. The solution of the solute in the first liquid is generally termed a feed solution and the second liquid is generally termed an extractant. In an undissolved state the solute can be a solid, a liquid, or a gas. The solute tends to distribute itself between the two liquids in accordance with the relative solubility of the solute in the two liquids when the feed solution is brought into contact with the liquid extractant. Since they are essentially immiscible, the feed solution and the liquid extractant form two distinct thermodynamic phases when in contact with one another. These two phases can be physically separated from one another, which effects a separation of a fraction of the solute from the feed solution.
Typically in conventional liquid-liquid extraction processes to promote a rapid distribution of the solute between a feed solution and an extractant, the feed solution and the extractant are mixed together intimately. Frequently, however, such intimate mixing gives rise to troublesome problems. For example, the mixing generally involves forming a dispersion of one of the liquids in the other. The resulting dispersion is frequently quite stable, so that it takes a long time for the dispersed liquid to coalesce. Consequently, the throughput of the solutetransfer process is undesirably low or the inventory of feed solution and extractant tied up in the process is undesirably high.
U.S. Pat. No. 3,956,112 to Lee et al. refers to an extraction process in which a porous membrane serves as a partition between two immiscible solvents. Solutes from one solvent are transferred to the other solvent by way of the porous membrane via direct solvent-solvent contact. In practice, however, conventional extraction processes in which immiscible solvents are separated by a porous membrane generally do not prevent one solvent from forming a dispersion in the other. One or the other solvent typically seeps through the porous membrane and becomes dispersed in the solvent on the other side of the membrane. Consequently, conventional extraction processes involving immiscible solvents separated by a porous membrane generally must provide a settling tank and a solvent return mechanism to coalesce the dispersion formed by the seepage of one of the solvents through the membrane and to return the solvent so recovered to its source.
In my U.S. patent application Ser. No. 644,895, filed Aug. 28, 1984 (the '895 application), I disclosed a solute transfer process in which a first side of a porous membrane is contacted with a feed solution and a second, opposing side of the membrane is contacted with a liquid extractant which is substantially immiscible with the feed solution. On the surface of the porous membrane either the feed solution tends to displace the extractant, or vice versa. The liquid which tends to displace the other defines a membrane-wetting liquid. The process of the '895 application further includes the step of maintaining a pressure difference across the membrane between the feed solution and the extractant. The pressure difference is imposed in a direction and a magnitude which is effective substantially to prevent the membrane-wetting liquid from flowing through the membrane and dispersing in the liquid on the other side. The interface between the feed solution and the extractant is thereby effectively immobilized at the porous membrane. The feed solution and the extractant come into contact through the pores of the membrane, permitting solute to be transferred through the pores from the feed solution to the extractant.
Although the solute-transfer process of the '895 application has proven satisfactory for many applications, there is room for improvement. In general the process requires a pressure-difference controller for maintaining a pressure difference across a porous membrane within a predetermined range to immobilize the interface between the feed solution and the extractant at the membrane. Suitable pressure-difference controllers tend to be expensive and prone to malfunction. Failure of the pressure difference controller to maintain the pressure difference in a required range generally results in seepage of the membrane-wetting liquid through the membrane. Such seepage generally results in a loss of the membrane-wetting liquid and concomittant contamination of the liquid separated from the membranewetting liquid by the membrane.